In the context of an internal combustion engine, internal combustion may be understood to particularly involve exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversation of chemical species. The fuel may generally comprise organic compounds, especially hydrocarbons (HC), while the oxidant may generally comprise oxygen (O2). With a proper stoichiometric ratio, complete combustion may be understood to result in the production of carbon dioxide (CO2) and water (H2O), as may be shown by the following reaction:CaHb+(a+b/4)(O2+3.773N2)=aCO2+(b/2)H2O+3.773(a+b/4)N2 
However, complete combustion is often difficult to achieve. Consequently, from incomplete or partial oxidation/combustion, as well as contaminates entering an engine cylinder (e.g. engine lubricating oil), exhaust emissions may include carbon-containing substances in non-solid form, such as liquid/vapor or gas form (e.g. hydrocarbon compounds from fuel and/or oil, and carbon monoxide), as well as carbon-containing substances in solid form (e.g. soot).
Particulate matter (PM) emitted from an internal combustion engine, such as a diesel engine, may be generally composed of volatile and solid fractions. The solid fraction may mainly comprise carbon and a small amount of inorganic ash. The carbon of soot has historically been referred to as being amorphous, but may better be considered polycrystalline or nanocrystalline materials of graphite or diamond within an amorphous carbon matrix.
The volatile fraction of particulate matter may consist of unburned and partially burned hydrocarbons (e.g. from fuel and lubricating oil), and sulfur compounds. Dry soot particles (i.e. solid) may be formed in the combustion chamber of an engine while most of the volatile fraction of the particulate matter enters the particle phase from the gas phase as the exhaust cools. The particle phase of the volatile fraction consists of precipitated liquid particles and precipitated liquid that attach to preexisting solid particles, which may be referred to as “wet” soot. Hence, particulate matter is a combined measure of solid and volatile fractions.
Depending upon operating conditions, an internal combustion engine may emit exhaust emissions containing a large amount of soot. The soot may cause adverse environmental and human health concerns. For example, with regards to environmental concerns, soot may contribute to global warming. As for human health concerns, soot may include carcinogenic substances and cause diseases affecting the human respiratory system. Furthermore, the U.S. Environmental Protection Agency (EPA) has set forth that engine exhaust may increase morbidity. As a result, the California Air Resource Board (CARB) has proposed a Low Emission Vehicle (LEV) III standard to further reduce gaseous and particulate matter emissions from light-duty and medium-duty vehicles. Black Carbon (BC), which may be a primary component of the soot, may be added for the carbon dioxide equivalency calculation.
To better study environmental and health concerns related to exhaust generated by internal combustion engines, there has been a need to be better characterize exhaust emissions, and more particularly soot emissions. Instruments have been developed to measure soot particle size distributions, soot particle surface areas, and soot particle mass emissions concentration. Furthermore, since mass-based particulate matter emissions remain an important criteria regulated by regulatory agencies, instruments which are able to measure soot mass emission in real-time are in demand by industry, regulatory agencies, and academic institutes.
Several instruments are commercially available for real-time soot particle mass emissions concentration determination. A photo-acoustic sensor is one technology which may be used for soot mass concentration determination. The photo-acoustic sensor may be understood to measure real-time soot particle mass emissions concentration with a photo-acoustic principle. A modulated laser beam heats soot particles, and then the soot particles are allowed to cool by turning off the laser beam. A sensitive microphone measures sound signals generated by the process of heating and cooling of the soot particles. Then, the sound signals measured by the microphone may be converted to a soot volume concentration. To ensure the accuracy of the measurement, it may be necessary to calibrate the instrument with soot particles by using a gravimetric filter method.
Laser Induced Incandescence (LII) is another major technology which may be used for real-time soot mass measurement from flames and internal combustion engines. A high intensity laser may be applied to heat soot particles to a temperature close to the soot particle sublimation temperature. Radiation of the soot particles during the cooling process is recorded. Then it is compared to a well defined cooling process which is obtained from a mathematic model. From there, the average diameter of the primary soot particle in soot clusters is obtained. Finally, soot volume may be calculated from the diameter of the primary particle and other parameters. To make this instrument work correctly, it may have to be calibrated against other soot measurement instruments or gravimetric filter measurement.
However, the foregoing technologies used for real-time soot mass measurement are not understood to provide a direct correlation to soot volume or mass. The conversion of a measured signal to the soot mass for those instruments is normally based on limited empirical data and many assumptions. Those empirical data and assumptions may be satisfactory under some conditions, but not all conditions. The foregoing technologies may be sensitive to engine technology, gas compositions, particle size distributions, etc. Moreover, since there are no particulate matter reference standards available, calibration procedures for these technologies may be complicated, time-consuming, and may produce different results among laboratories. Furthermore, calibration may require that the operator have a good technical background for the instrument and aerosol science. Additionally, soot mass results determined with these technologies may be questioned in that the instruments may not be calibrated as frequently as may be required, particularly in light of difficult calibration procedures.
In light of the above, an apparatus to determine total and solid carbon content, that may be easy to calibrate using traceable devices and commonly used standard operating procedures, is needed to overcome the aforementioned difficulties in the art.